Cast refractory products



Patented Apr. 1 2 1949 2,467,122 CAs'rnEFRAC'rORY PRODUCTS Theodore Estes Field, Louisville, Ky., as'sigrior .to' Cor-hart Refractories Company, Louisville, Ky, a corporation of Delaware No Drawing. Application April 10, 1945, Serial No. 587,614

The tremendous increase in production of mag,- nesium and its alloys has emphasized the unsatisfactory performance of presently available refractories for melting and containing the molten metals. It is an object of this invention to disclose a novel and improved heat cast refractory for this and similar purposes. By heat cast is meant the complete melting of the ingredients as for example with the techniques disclosed in U. S. Patent #l,615,750 to Fulcher, and shaping into the desired form by casting into molds and solidifying.

Magnesium is an extremely reactive metal, and its attack is apparently chemical, resulting in the formation of MgO and the metal of the refrac- 9 Claims. (01. 10662) tory oxide. Large well developed crystals such as are obtained by heat casting as opposed to the usual burning minimize the surface area exposed to attack. Because of its readycastability in small shapes and relatively high resistance to spalling for a cast refractory, soda beta alumina (ideally NazOllAlzOs) castings have been cl-- p-loyed with considerable success. Such castings unlike most heat cast refractories are apt to have considerable porosity, so much so in fact that the normal pipe resulting from the change in volume from liquid to crystal is often completely absent, this volume change being distributed as pores between the platy crystals. Any porosity of course is a disadvantage since it permits penetration of the metal with an increase in area of attack. Furthermore reaction in the interior of a refractory is particularly destructive since it often occurs with an increase in volume tending to disrupt the refractory. The resistance of beta alumina castings to spalling is apparently associated with the low elastic constants resulting from the loose jointed, platy crystallization.

In another application I have disclosed that well developed crystals of magnesiumspinel (NIgO.Al203) are very resistant to attack by magnesium. Theoretically the MgO part of the coinposition is not reducible by Mg at all.

In their original investigations of the high temperature phases of the MgO-AlzOs system, Rankin and Merwin found that abeta alumina phase occurred at times and concluded that it was monotropic since its appearance could not be predicted. I have discovered however that the beta alumina which is produced with NazO added is perfectly stable in the presence of spinel and can be produced at will. In other words the MgO does not cause the alumina to crystallize as corundum but instead the alumina divides up between the MgO forming MgQAlzOs and the No.20

forming NazOllAlzOs and only in the case of a deficiency of MgO and. NazO does corundum occur as an additional crystal phase.

Since spinel and beta alumina are thus discovered to be mutually compatible it possible to make by combining the two phases, a cast refractory which may be considered either a. beta alumina refractory with improved chemical resistance due to the presence of spinel or a spinel refractory with improved spalli resistance due to the presence of the beta alumina. Since the problem of chemical resistance is usually the most serious I normally prefer spinel phase as major ingredient but by cha 11g the proportions a graduation in properties can be obtained. While the property or chemical resistance is more or less additive is not the case with theelastic properties presumably because of the interlocking of crystals which occurs when two or more different phases are present. Nevertheless it may be stated qualitar tively that the resistance to spelling increases as the betal alumina proportion increases.

For the best castings, I preferto use thepure alumina commonly used for metal production by electrolysis. A good grade of calcined .magnesite low in silica can be used forthe MgO and soda ash may be used to supplyvthe NazO in which form the .NazO is introduced .more cheaplythan as sodium aluminate. While K20 forms an analogous beta alumina phase, potassium sources are normally more expensive and its use offers no outstanding advantages. In the absence of MgO, the addition of some 6 or 7% S102 suppresses the beta alumina crystallization and yields instead alpha alumina (corundum) and a silicous glass containing the NazO and saturation amounts of A1203. In the presence of MgO, the effect of "S102 is much more pronouncedand in a melt with only 4.5% S102 the beta alumina phase almost vanished although sufficient lifa;0 was present to theoretically form 5 %75 at this phase. In this system as silica is added-the beta alumina decreases and besides the silicone phase, spinel is formed withAlzOs :insolidsolution. To gain anybenefit ofia beta alumina phase. it is therefore essential to keep silica at a minimum. Since the raw materials are completely melted however it is obvious that any combination of raw materials which will yield the desired chemical analysis can be used.

Compositions in this system which have given good cast refractories are illustrated in Table I.

Table I Calculated Phases Melt A1203 MgO Na20 020 S10, K20

- Beta spme] alumina A 77.0 21.2 1.3 .3 .2 75 25 B 83.0 14.0 2.7 .2 .1 so 50 o 88.6 7.2 4.0 .1 .1 25 75 1) 82.4 14.0 1.3 .2 .1 2.0 50 50 In this system spinel crystallization takes precedence over the beta alumina crystallization. Thus when 28% MgO and 2.4% Na20 were added to A1203, spinel was the dominant crystal phase and no beta alumina was found petrographically. It is therefore essential to keep the MgO down to that required for the desired amount of spinel as well as to add the proper amount of Na20 for the beta alumina.

If less than enough MgO is present to con-- vert to spinel all A1203 not required for the beta alumina, acorundum phase might be expected. Actually if appreciable beta alumina is present, the excess A1203 will be almost completely taken into solid solution with the spinel. This phase of solid solution of A1203 in Mg0.Al20a is also quite resistant to chemical attack by magnesium and its formation is not necessarily to be avoided. Examples of good compositions in this system are shown in Table II.

In melt E there is more A1203 than can be taken into solution by the spinel, and corundum is also present as a phase.

Since in spinel the A1203 percentage is 2.5 times the MgO and since in beta alumina the A1203 is 18.0 times the N920 or 11.9 times the K20, an excess of flux can be avoided by using A1 03 in at least these proportions to the actual fluxes .present. An excess of alkali metal oxide is particularly harmful to corrosion by magnesium when S102 is present. If it is desirable to use a batch containing both Na20 and K20, an excess of alkali can be avoided by calculating the alumina required by each separately.

By principally in the following claims I mean over 95% of the total composition.

What I claim is 1. A heat cast refractory composed principally of beta alumina and magnesium spinel and in which the beta alumina lies between 20% and 80% and in which alkali oxide is present in an amount at least six times that of silica.

2. A heat cast refractory composed principally of beta alumina and magnesium spinel and in which the magnesium spinel lies between 50% and 80% and in which alkali oxide is present in an amount at least six times that of silica.

3. A heat cast refractory composed principally of 20% to beta alumina and a solid solution of alumina in magnesium spinel and. in which alkali oxide is present in an amount at least six times that of silica.

4. A heat cast refractory composed principally of 20% to 50% beta alumina, corundum and a solid solution of alumina in magnesium spinel and in which the MgO lies between 5.5% and 22.5% by weight by chemical analysis and in which alkali oxide is present in an amount at least six times that of silica.

5. A heat cast refractory composed principally of A1203 and containing 5.5% to 22.5% MgO and 1% to 4% Na20 by weight by chemical analysis in which the percentage of A1203 is not less than the sum of 2.5 times the percentage of MgO plus 18.0 times the percentage of Na20 and in which alkali oxide is present in an amount at least six times that of silica.

6. A heat cast refractory composed principally of A1203 and containing 5.5% to 22.5% MgO, 1% to 4.2% N220 and 0% to 5% Si02 by weight by chemical analysis and in which the percentage of A1203 is not less than the sum of 2.5 times the percentage of MgO plus 18.0 times the percentage of Na20 and in which alkali oxide is present in an amount at least six times that of silica.

7. A heat cast refractory composed principally of A1203 and containing 5.5% to 22.5% MgO and 1.5% to 6.2% K20 by weight by chemical analysis and in which the percentage of A1203 is not less than the sum of 2.5 times the percentage of MgO plus 11.9 times the percentage of K20 and in which alkali oxide is present in an amount at least six times that of silica.

8. A heat cast refractory composed principally of A1203 and containing 5.5% to 22.5% MgO, 1.5% to 6.2% K20 and 0% to 5% S102 by weight by chemical analysis and in which the percentage of A1203 is not less than the sum of 2.5 times the percentage of MgO plus 11.9 times the percentage of K20 and in which alkali oxide is present in an amount at least six times that of silica.

9. A heat cast refractory composed principally of A1203 and containing 5.5% to 22.5% MgO, 0% to 5% Si02 and 1% to 6% of a group consisting of Na20 and K20 and in which the percentage of A1203 is not less than the sum of 2.5 times the percentage of MgO plus 11.9 times the percentage of K20 plus 18.0 times the percentage of Na20 and in which alkali oxide is present in an amount at least six times that of silica.

THEODORE ESTES FIELD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,075,694 Benner et al Mar. 30, 1937 2,154,069 Fessler et al Apr. 11, 1939 2,261,639 Benner et a1 Nov. 4, 1941 

